1. Field of the Invention
The present invention relates to a method of manufacturing a hexagonal ferrite magnetic particle, and more particularly, to a method of manufacturing a hexagonal ferrite magnetic particle that is suitable for use in manufacturing a particulate magnetic recording medium by a glass crystallization method.
The present invention further relates to a hexagonal ferrite magnetic particle obtained by the above method, and a particulate magnetic recording medium comprising the above hexagonal ferrite magnetic particle.
2. Discussion of the Background
Recently, ferromagnetic metal particles have come to be primarily employed in the magnetic layers of magnetic recording media for high-density recording. Ferromagnetic metal particles are comprised of acicular particles of mainly iron, and are employed in magnetic recording media for various applications in which minute particle size and high coercive force are required for high-density recording.
With the increase in the quantity of information being recorded, magnetic recording media are required to achieve ever higher recording densities. However, in improving the ferromagnetic metal particle to achieve higher density recording, limits have begun to appear. By contrast, hexagonal ferrite magnetic particles have a coercive force that is high enough for use in permanently magnetic materials. Magnetic anisotropy, which is the basis of coercive force, derives from a crystalline structure. Thus, high coercive force can be maintained even when the particle size is reduced. Further, magnetic recording media employing hexagonal ferrite magnetic powder in the magnetic layers thereof can afford good high-density characteristics due to their vertical components. Thus, hexagonal ferrite magnetic particle is an optimal ferromagnetic material for achieving high density.
In recent years, coating the surface of hexagonal ferrite particles with Al (see Document 1 (Japanese Unexamined Patent Publication (KOKAI) Showa No. 58-19733), Document 2 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 5-283219) and Document 3 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 9-213513)) and incorporating a small quantity of Mg or Al into hexagonal ferrite magnetic particles (see Document 4 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-337521)) have been proposed to further improve hexagonal ferrite magnetic particles having the above-described good characteristics. The contents of Documents 1 to 4 above are expressly incorporated herein by reference in their entirety.
The techniques described in Documents 1 to 4 are all attempts to enhance dispersion and achieve a uniform particle size distribution so as to increase the suitability of hexagonal ferrite magnetic particles to magnetic recording applications.
However, research conducted by the present inventors has revealed that medium output decreases in magnetic recording media employing the hexagonal ferrite magnetic particles surface-coated with Al or the like that are described in Documents 1 to 3. This has been attributed to large amounts of Fe and Ba dissolving out of the particles due to non-uniformity of the coating of Al or the like, with these ion components then reacting with fatty acids or their derivatives (fatty acid esters and the like) to form fatty acid metal salts that contaminate the head. Research by the present inventors has also revealed that these particles cause considerable abrasion of dispersion media during the preparation of coating liquids for magnetic recording media. The abrasion of dispersion media is linked to increased cost due to reduced medium service life. There may be a risk of powder produced by medium abrasion contaminating the medium and compromising magnetic characteristics.
Additionally, the hexagonal ferrite magnetic particles described in Document 4 are of large diameter and cannot be employed as magnetic particles in the magnetic recording media in which higher densities are currently required.